The fuels for diesel motors are designed to auto-ignite after being injected in highly compressed air. They generally consist of a mixture of various constituents in proportions such that the fuel has certain qualities and meets certain related specifications, for example, cold resistance, sulfur content and value of cetane.
In the refining plants, such a fuel is obtained through a technique called in line mixing, by which the various liquid constituents, as well as additives, are introduced simultaneously and continuously in one duct serving as a mixer. The flow rate of the various constituents is controlled and operated by computer and the preparation time of one batch of fuel can take up to 30 hours.
Naturally, the properties of the resulting product are monitored several times during the making and analyses are completed on samples taken from the outflow of the mixer, or from the: storage tank during the filling. From the results of theses analyses, the flow rates of the mixture's, constituents and of the additives are adjusted to align the measured values with the set values.
Of course, for each analysis, there is an advantage to limiting as much as possible the time periods between the time when the sample of the mixture during preparation was taken and the time the measured value was obtained, and, in general, the analyzers used for this procedure, called in line analyzers, meet this requirement.
However, one of the most important properties of diesel motors fuels, namely their capacity to auto ignite, is not submitted to an on line control as it exits the mixer because of the difficulties involved in measuring this characteristic. It is defined as the time between the injection of fuel and its ignition in a combustion chamber, under conditions set by the ASTM D 613 standards, where the unit of measurement is conventionally called the cetane number.
According to the conditions of measurement, the value 15 of this cetane number is attributed, to heptamethylnonane, whereas the value 100 corresponds to the combustion under the same conditions of normal cetane (normal hexadecane). An intermediary CN number (Cetane Number, attributed to a fuel means that the latter has an auto ignition equal to that of a mixture of heptamethylnonane and normal cetane such that: EQU CN=x+0.15(100-x)
where x is the cetane content (% per volume) of the heptamethylnonane and normal cetane mixture.
The measuring, long and complicated, takes place in very precise conditions set by the ASTM D 613 standard, with the help of a device defined in the same standard. This device contains a diesel motor adjusted by the Corporation Fuel Research company and universally known in the field under the name of "C.F.R. motor". However, the measurement procedure with this device is lengthy (approximately 1 hour), delicate and requires many manual interventions.
Methods and apparatuses have been perfected in order to reduce the measuring time while still automatically generating the measurement in question. In particular, the application for European patent EP-A-0 610 118, which relates to the procedure as described in the SAE 890419 document, proposes a method and apparatus for continuously measuring the cetane index. If these methods anti apparatuses enhance the method for measuring a fuel's cetane, they nevertheless have some drawbacks, such as the use of common circuits and capacities for the standard product and the fuel sample during manufacturing, as well as the need to use many electrovalves with their pilot systems,
There are other methods for measuring the auto ignition delay of a fuel, such as the calculation of the cetane number from easily measured properties, such as density or distillation characteristics. However, the results obtained are not very reliable and do not take into account the frequent introduction in certain types of fuel of a procetane additive designed to increase, if necessary, the cetane number of the mixture. It is interesting to note, along these lines, that the CFI motor does in fact take the effect of the procetane additive into account in its measurements.
There is therefore a need, in this technique for a fast, reliable and automatic measurement system, used in line during the preparation procedure of a fuel for diesel motors, that allows to better control the auto ignition characteristic represented by the cetane number.